As we have seen in the New Chapters in One Piece Zoro has been cut on his left eye! And there has been many theory's on how it's going to affect him!
I will give the 2 most popular theory's first and then mine
1.Demon(ONI)Eye because of his most used attacks (Oni Giri (鬼斬り Oni Giri?, literally meaning "Demon Slash") and because of his fight between t-bone when his left eye glows red! And the reason that its cut is because if he opens it could hurt Zoro or Zoro is trying to have more of a challenge when fighting!(Some type of Kenpachi Eye Patch thing bleach refrence)
2.The eye literally does nothing and the only new technique that he has is Haki!
3.My personal theory! I think that Zoro's left eye is shut! BUT here me out here one of the few techniques we have seen from Zoro is the Asura Demon!
The Asura Demon comes from Buddhism and Hinduism cultures! Well in Buddhism text we see that the Asura demon is a sort of Demi-God that becomes from the six human paths of rebirth
(Six paths of Pain yes I know)
Asura demons have been know to have more limbs and other organs like EYES!
What I'm trying to say is Zoro has Awakenes the Third Eye
Or in Buddhist culture The heavenly eye that has replaced his left eye or has added one on his forehead!!!
"The heavenly eye penetrates what is without obstruction"-Buddhist text
The Heavenly Eye is said to see defects and weaknesses in ANYTHING as in Buddhist culture states! That's means he will be able to work around tough enemies SEE weak spots and cut through tougher material than STEEL!!!
My reason why I think this is true one of them being that Zoro no longer has the bandana over his head! Because Daz Bones mentioned how Zoro was stronger with his bandana on(Covering fore-head where The Heavenly Eye is supposed to be located in Buddhist culture).
What I'm trying to go in more on this topic I think Zoro's new eye will be located on his fore-head or replaced his left eye
And the Heavenly Eye is technically not a demon eye it is an eye that has been given through enlightenment and training ones mind body and soul!!!